// Judges with GCC >= 12 only needs Ofast
// #pragma GCC optimize("O3,no-stack-protector,fast-math,unroll-loops,tree-vectorize")
// MLE optimization
// #pragma GCC optimize("conserve-stack")
// Old judges
// #pragma GCC target("sse4.2,popcnt,lzcnt,abm,mmx,fma,bmi,bmi2")
// New judges. Test with assert(__builtin_cpu_supports("avx2"));
// #pragma GCC target("avx2,popcnt,lzcnt,abm,bmi,bmi2,fma,tune=native")
// Atcoder
// #pragma GCC target("avx2,popcnt,lzcnt,abm,bmi,bmi2,fma")
/*
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
typedef tree<int,null_type,less<int>,rb_tree_tag,tree_order_statistics_node_update> ordered_set;
- insert(x),erase(x)
- find_by_order(k): return iterator to the k-th smallest element
- order_of_key(x): the number of elements that are strictly smaller
*/
#include<bits/stdc++.h>
using namespace std;
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
uniform_real_distribution<> pp(0.0,1.0);
#define int long long
#define ld double
#define pii pair<int,int>
#define piii pair<pii,int>
#define mpp make_pair
#define fi first
#define se second
const int inf=1e9;
const int mod=1e9+7;
const int maxn=505;
const int bl=650;
const int maxs=655;
const int maxm=200005;
const int maxq=500005;
const int maxl=20;
const int maxa=250000;
const int root=3;
int power(int a,int n){
int res=1;
while(n){
if(n&1) res=res*a%mod;
a=a*a%mod;n>>=1;
}
return res;
}
const int iroot=power(3,mod-2);
const int base=101;
ld dp[maxn][maxn];
void solve(){
int n,k;cin >> n >> k;
vector<pii> p(n);
for(int i=0;i<n;i++) cin >> p[i].se >> p[i].fi,p[i].fi=(p[i].fi==-1?inf:p[i].fi);
sort(p.begin(),p.end());
ld res=inf;
for(int d=0;d<=k;d++){
for(int i=0;i<=n;i++) for(int j=0;j<=k-d;j++) dp[i][j]=inf;
dp[0][0]=0;
for(int i=0;i<n;i++){
for(int j=0;j<=k-d;j++){
if(j<k-d) dp[i+1][j+1]=min(dp[i+1][j+1],dp[i][j]+(ld)p[i].se/(d+1));
dp[i+1][j]=min(dp[i+1][j],dp[i][j]+(i-j<d?ld(p[i].fi)/(i-j+1):0));
}
}
res=min(res,dp[n][k-d]);
}
cout << setprecision(10) << fixed << res << '\n';
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);cout.tie(NULL);
int test=1;//cin >> test;
while(test--) solve();
}
